1. Field of the Invention
This invention relates generally to systems for driving strings of LEDs, and more particularly to methods of facilitating ‘local dimming’ in a display device made from LED strings.
2. Description of the Related Art
Light-emitting diodes (LEDs) are becoming increasingly popular as a light source. In some applications, such as providing a source of backlight for a display device, many LEDs are used. The LEDs are typically connected in series, cathode-to-anode, to form an LED ‘string’, with all of the LEDs in the string driven on by applying a voltage between the first anode and the last cathode in the string. Since having a large number of LEDs in a string would necessitate a corresponding large driving voltage, the LEDs are typically arranged into a number of smaller strings, each of which can be driven on with a lower driving voltage.
A simplified illustration of such a system is shown in FIG. 1. Here, an LED drive circuit 10 interfaces with a number of LED strings 12, 14, 16, each of which includes multiple LEDs connected in series. Drive circuit 10 also controls a number of switching elements 20 which are connected to the cathode ends of respective strings, and provides a driving voltage V+ which is applied to the anode end of each string. In this example, a timing circuit 22 within LED drive circuit 10 operates the switching elements 20 in unison to pulse-width modulate (PWM) the currents conducted by the LED strings to which they are connected; the duty cycle of the PWM signals determines the brightness of the LEDs in the strings. The DC voltage provided to the LED strings may be provided by, for example, a switching power converter (not shown)—most typically a boost-type power converter (referred to herein as a ‘boost converter’, which produces an output referred to herein as a ‘boost voltage’)—or a charge pump boost circuit.
This brightness control method can have several drawbacks. Assume, for example, that there are 8 LED strings, each of which conducts 20 ma of current when on. Thus, when all 8 strings are driven on, the total current load jumps from 0 ma to about 160 ma. A timing diagram illustrating the individual and summed pulse-width modulated (PWM'd) currents for the 8 strings is shown in FIG. 2.
If the LED drive circuit includes a DC/DC boost converter which receives a DC input voltage Vin of 12V and produces a DC output voltage V+ of 36V, the average value of the summed inductor currents (i.e., the converter's input current) will be about (160 ma×3=) 480 mA when the LEDs are on. Thus, the input current suffers from large fluctuations due to the periodic and simultaneous on/off operation of the LED strings. In battery-operated system such as a laptop, Vin may decrease over time to, for example, 6V, and the input current change will be increased accordingly.
One approach that has been taken to reduce the magnitude of load fluctations of this sort is to phase shift the PWM'd currents so that they are evenly spaced throughout the switching cycle. This technique may be effective at making the total current load nearly constant, but requires that the ON times for each LED string be identical, which may be unacceptably limiting.
Some display devices employ a technique known as ‘local dimming’, in which the display's screen area is divided into a number of areas, with the brightness of the backlight behind each area being independently controllable. This can provide a higher contrast ratio for the screen, as well as lower its power consumption. To provide this functionality, the individual LED strings need to be independently controllable. However, this technique can cause a complicated load condition to be presented to the LED drive circuit. This situation is illustrated in FIG. 3. Since there is independent control of each LED string, the PWM'd current of each string can be different, which may result in the total current load varying widely as shown. The different currents conducted by the respective strings results in the voltage drop across each string also being different, which can make it difficult for the boost converter output voltage to provide proper headroom control for the LED strings.